Grid stiffened structures provide favorable rigidity characteristics in resisting buckling from a compression load and/or resist hoop stress under internal pressure load. Grid stiffened structures typically include webs, walls or shells, hereinafter referred to as wall, supported by a grid lattice of reinforcement members, such as ribs, stiffeners or stringers, which are positioned across the wall. These stiffeners or stringers form a boundary shape on the wall to provide desired structural reinforcement. One example of a grid stiffened structure is an iso-grid structure. The iso-grid structure has the reinforcement stiffeners or ribs form a triangular boundary shape formation on the wall. The triangular pattern of ribs or stiffeners is efficient and retains rigidity while saving material and weight. The iso-grid structure acts much like an isotropic material with equal properties measured in any direction.
Another example of a grid stiffened structure, which includes a rectangular boundary shape formed by ribs or reinforcement members which extends across the wall, is referred to as an ortho-grid structure. An ortho-grid structure is a variation of an iso-grid structure and is not isotropic but has different properties than triangular boundary shaped pattern of stiffeners or ribs positioned across the wall. Ortho-grid structures are not isotropic but have different properties from different angles but in certain applications can be used in place of iso-grid structures. Other grid stiffened structures are available which have stiffeners or ribs forming other boundary shaped configurations which are positioned across the wall structure being reinforced.
Use of subtractive manufacturing, where a metal sheet, for example, is machined to remove material and to form grid reinforcement formations on a wall is an expensive method of manufacturing. Other expensive fabrication techniques can be employed which would include fabricating the stiffeners or ribs separately and apart from the wall structure and welding the separate reinforcement members to the wall. Additional expensive methods have been used such as utilizing composite material which includes expensive tooling.
Grid stiffened structures which provide stiff, high strength and light weight constructions have been expensive to fabricate and as a result have been limited for use in general aerospace applications which experience takeoffs, cruising and landings and spaceflight applications which experience launch, station keeping, entry, descent and landings. With grid stiffened structures, having stiffness, high strength and lightweight characteristics along with providing a sealed wall to an outside of a structure, these structures have been useful for application with such items as a pressurized propellant tank for rockets and which are advantageous over a monocoque construction.
As a result, there is a chief interest in constructing grid stiffened structures which provide needed stiffness, high strength and lightweight characteristics for structures to resist buckling from compression loads and resist hoop stress pressure loads at a lower cost than currently is incurred in fabricating these grid stiffened structures. Typically, as mentioned, subtractive manufacturing measures are used where sheet metal is milled, portions are formed or otherwise hammered and welded in fabricating the grid stiffened structures. Composite constructions have been used but include expensive tooling, as previously mentioned. There is a need to achieve less expensive fabrication processes for these stiffened grid structures with the use of Additive Manufacturing (“AM”) or otherwise referred to as 3-D printing so as to avoid expensive milling, welding and/or tooling.
However, there are limitations with use of AM with use of 3-D printing fabrication of structures, in particular, with respect to overhang of the material being applied to form the structure with respect to a vertical axis. For example, where the structure will itself have a vertical or upright section, such as with a more elongated pressurized propellant vessel for a rocket, the ribs or stiffeners that would form a square or rectangular boundary shape with respect to the wall could not be AM fabricated. If the rib structure was accessible the use of expensive removable support structures (i.e. breakdown tooling) for constructing the rib would be required.
The installation of the support structures and dismantling of the support structures if practical or even possible along with the employment of an expensive tilt-table assembly if needed, would require an additional expense in fabrication of, for example, a pressurized propellant vessel. There is a need to utilize conventional AM printing methods be able to fabricate grid stiffened structures in a vertical direction so as to construct, for example, pressurized vessels or portions thereof. Vessels that will resist aforementioned buckling compressive force and hoop stress pressure without employment of costly steps associated with such 3-D printing or otherwise avoiding use of printing a flat sheet and rolling the sheet.